Development of small information equipment has been rapidly advanced miniaturization of its electronic components mounted thereon in recent years. Ball grid allay (hereinafter, referred to as “BGA”) has been applied to the electronic components so that electrodes are arranged on a rear surface thereof, in order to cope with narrowing of the terminals and/or reduced size of mounting area according to downsizing requirement.
As the electronic component to which BGA is applied, a semiconductor package is exemplified. In the semiconductor package, a semiconductor chip having electrodes is sealed by any resins. On the electrodes of the semiconductor chip, solder bumps are formed. Each solder bump is formed by connecting a solder ball with the electrode of the semiconductor chip. The semiconductor package to which BGA is applied is mounted on a printed circuit board by putting each solder bump on the printed circuit board so that each solder bump can contact an electrically conductive land of the printed circuit board and connecting the solder bump fused by heating with the land. Further, in order to cope with any higher density mounting requirement, a three dimensional high density mounting structure in which the semiconductor packages are piled along a height direction thereof has been studied.
However, when BGA is applied to the semiconductor packages on which the three dimensional high density mounting is performed, each ball becomes flat by weight of the semiconductor packages, which causes a short-circuit between the electrodes. This constitutes any hindrance to the high density mounting performance.
Accordingly, a solder bump in which Cu ball is connected to an electrode of an electronic component through paste has been studied. The solder bump having the Cu ball can support the semiconductor package by the Cu ball which does not melt at a melting point of the solder when mounting the electronic component on the printed circuit board even if the weight of the semiconductor packages is applied to the solder bump. Therefore, it is impossible for the solder bump to become flat by the weight of the semiconductor packages. As a related art, patent document 1 is exemplified.
By the way, the miniaturization of electronic components allows the high density mounting structure but such high density mounting structure has caused any software errors. These software errors may occur by rewriting contents stored in a memory cell of a semiconductor integrated circuit (hereinafter, referred to as “IC”) when entering α ray into the memory cell. It is considered that the α ray radiates by α decay of a radioactive isotope such as U, Th and 210Po in a solder alloy. Accordingly, a solder material decreasing radioactive isotope content and having less α ray has been developed in recent years.
For this reason, the Cu ball disclosed in the patent document 1 is also required to decrease any software errors occurred in the high density mounting structure.